Magnetic refrigeration technology at ambient temperature has been known for more than twenty years and the advantages it provides in terms of ecology and sustainable development are widely acknowledged. Its limits in terms of its useful calorific output and its efficiency are also well known. Consequently, all the research undertaken in this field tends to improve the performances of such a generator, by adjusting the various parameters, such as the magnetization power, the performances of the magnetocaloric element, the heat exchange surface between the heat transfer fluid and the magnetocaloric elements, the performances of the heat exchangers, etc.
The choice of the magnetocaloric materials is determining and influences directly the performances of a magnetocaloric thermal generator. To increase these performances, a solution consists in associating several magnetocaloric materials having different Curie temperatures in order to increase the temperature gradient between the ends of this assembly.
Thermal generators are thus known, which comprise at least one thermal module M such as the one represented on FIGS. 1A and 1B and comprising magnetocaloric materials MC arranged side by side and aligned, and circulating means for the heat transfer fluid such as pistons P, intended for giving the heat transfer fluid a reciprocating movement so as to pass through the set of magnetocaloric materials MC, to either side of the latter, between the cold side F and the hot side C of magnetocaloric materials assembly MC, and synchronized with the variation of a (not represented) magnetic field. As shown on FIGS. 1A and 1B, these pistons P are arranged on both sides of magnetocaloric materials assembly MC and move alternately in one direction and in the other, FIGS. 1A and 1B representing the pistons in their two extreme positions.
It appears in FIGS. 1A and 1B that the fluid moves either in one direction, towards hot end C (the direction of movement of the heat transfer fluid is shown by the dotted arrows, see FIG. 1A) when the magnetocaloric materials are exposed to a heating cycle, or in the other direction, towards cold end F (the direction of movement of the heat transfer fluid is shown by the solid arrows, see FIG. 1B) when the magnetocaloric materials are exposed to a cooling cycle.
This thermal module M has a disadvantage due to the fact that, in order to reach a temperature gradient, it is necessary to circulate a heat transfer fluid through all of the materials. The use of several magnetocaloric elements MC leads to an increase of the material length to be crossed by said heat transfer fluid. Thus, in order not to reduce the number of cycles (a cycle being defined by a heating and a cooling of the magnetocaloric element), it is necessary to increase the speed of the heat transfer fluid. Now, the increase of the speed leads to an increase of the pressure, which worsens the head losses and reduces the efficiency of the heat exchange between the heat transfer fluid and the magnetocaloric elements, and leads to a reduction of the thermal efficiency of the magnetocaloric generator.
It is also known that, in order to increase the thermal output of a magnetocaloric generator, a possibility consists in increasing the number of cycles per second. Now, this results in an increase of the speed, which also leads to the above-mentioned disadvantages.
A generator comprising a thermal module M such as illustrated in FIGS. 1A and 1B requires a non-negligible previous operating time in order to reach an exploitable temperature gradient between the two ends, because of the multiplicity of the materials used.
The applicant offered, in his not yet published patent application FR 08/05901, a magnetocaloric thermal generator allowing to improve the thermal efficiency of the known generators, with a same quantity or length of material.
It also offered, in patent applications WO 2007/026062 and WO 2008/012411, magnetocaloric thermal generators with a modular construction and comprising two distinct hot and cold circuits in contact with the magnetocaloric materials.